Field of the Invention
The invention relates to an apparatus for the conversion of analog audio signals to a digital data stream.
In order to widen the dynamic range of analog/digital converters (A/D converters) that are available in audio technology, a converter is used by means of the so-called "gain-ranging" technique in each of two parallel signal paths, the one serving for high levels and the other for low levels of the audio signal that is to be converted. The audio signal to be converted is amplified analogically in one path before conversion, and amplified in the other path by the same amount, but digitally after the A/D conversion. Both paths terminate in a double-throw switch which prepares for further processing the digital data stream from whichever path is optimally modulated by the audio signal that is to be converted.
Apparatus of this technology generally have the disadvantage that the switching from one path to the other for the processing of the digital data stream can make undesired clicks, distortion and noise modulation audible. Above all, circuit noise, DC offsets and gain errors in the circuits largely nullify the advantage of the dynamic gain. Attempts are made to minimize these disturbances by suppressing their subjective audibility.
A soft smoothing over in the switching from one path to the other has the considerable disadvantage that the data streams have to be delayed by the amount of the smoothing time, which in most cases is not acceptable.
The use of high-pass filters can reduce clicking, but it is a very complicated measure, inasmuch as it has to be implemented in both paths. The final accuracy of the digital high-pass filters used also leads to additional distortion.
To time the switching from one path to the other the digital information at the output of the converters has been used. Consequently the HF damping of the anti-aliasing filters leads to a falsification of the actual level at the entrance to the path, and in the case of high-level and high-frequency signals it can bring the result of switching erroneously to the output of the converter for low-level signals, although the latter is already over-modulated. The resultant distortions are completely unacceptable.